Aspects of this disclosure relate to the preparation of a stucco slurry useful in the manufacture of gypsum wallboard. In particular, one or more aspects of the disclosure generally relate to the use of sacrificial agents to reduce the disruptive effect of certain contaminants in a stucco slurry used in the production of gypsum wallboard and to the resulting wallboard product.
Gypsum wallboard (also known as drywall, plasterboard, sheetrock, or gypsum board) is commonly used as a construction material. Gypsum wallboard is widely used in building constructions primarily as interior walls and ceilings. Based on the type of construction being performed it may be desirable for the gypsum wallboard to exhibit certain performance properties, including being light weight with structural stability, as well as having reduced sound transmission, thermal resistance, impact resistance, fire resistance, mold and/or mildew resistance, sag resistance, and/or the like. Although it is desirable to have gypsum wallboard exhibit all or some of these properties, it is additionally desirable that the production cost of producing gypsum wallboard with such improved properties not be increased to any significant extent, as gypsum wallboard is commonly viewed as a commodity product.
Gypsum wallboard is commercially produced by depositing an aqueous slurry of stucco (also known as calcined gypsum and chemically identified as calcium sulfate hemihydrate), to form a core layer between two cover sheets, allowing the slurry to set (harden) and then drying the resultant product. Water is added to the stucco in an amount sufficient to hydrate the calcined gypsum and to produce a fluid slurry. Any water added beyond the amount required to re-hydrate the stucco, however, eventually must be removed from the slurry as the board is manufactured. The need to remove this excess water during wallboard production accordingly increases energy costs. In addition, board strength also is known to be inversely proportional to the amount of water used in manufacturing the wallboard. Thus, there is an incentive to reducing the amount of water used when preparing the stucco slurry.
Historically, dispersants or water reducers have been used as a stucco slurry additive as a way of minimizing the amount of excess water necessary for producing a desired amount of fluidity in the stucco slurry. Exemplary stucco dispersants (water-reducing agents) include, but are not limited to, lignosulfonates, polycarboxylates, naphthalene sulfonates, and combinations thereof. These and other dispersants enhance the fluidity of the stucco slurry by reducing the slurry viscosity. As a result of the reduced water demand obtained through the use of these dispersants, overall process control is improved and energy costs are limited. The improved slurry fluidity also promotes increased penetration of the cover sheets (e.g., enhances cover (paper face) penetration) by the stucco slurry resulting in a more structurally stable wallboard product. Still other additives, like foam (soap) may also be added to the stucco slurry to obtain/improve other wallboard properties.
Gypsum used in the manufacture of wallboard is obtained primarily from mining natural gypsum and from gypsum obtained by desulfurization of flue gas from a coal-fired or lignite-fired power plant (this synthetic gypsum is also called desulfogypsum (DSG) or flue gas desulfurization (FGD) gypsum). Regardless of the gypsum source, the gypsum that is calcined and used to prepare the raw stucco may contain a variety of contaminants. Such contaminants may include clays (common to mined gypsum) and fly ash (common to desulfogypsum). At least some of the contaminants present in the gypsum that is calcined to form stucco typically find their way into the stucco slurry. Such contaminants tend to interfere with the effectiveness of many of the commonly used additives included in the stucco slurry, particularly those additives used in small amounts, such as dispersants (water reducers) used to enhance the fluidity of the stucco slurry and foaming agents (soap) used to introduce voids into the set gypsum core for reducing the weight of individual boards.
In accordance with the present invention, to more efficiently manufacture gypsum wallboard certain sacrificial agents may be added to the stucco slurry to interfere with the action of these common stucco contaminants.